Electric furnace steel is produced in steelmaking furnaces powered by electricity. It offers advantages such as low investment, short construction periods, flexible production organization, and significant environmental, emission, energy, and carbon reduction benefits. Depending on the type of steelmaking furnace, electric furnace steel can be categorized as electric arc furnace steel, non-vacuum induction furnace steel, vacuum induction furnace steel, and electroslag furnace steel.
1. What are the charging methods for electric arc furnace?
When the EAF charge contains more than 35% ferrous iron (SRI), adding scrap from a basket can cause the SRI to adhere to the furnace walls. Advanced EAF manufacturers typically use a continuous charging system to add SRI to the furnace. This system uses a vibrating feeder to deliver the SRI to a belt conveyor, which then transports the SRI to a high-level hopper in the raw material bay for use in EAF steelmaking. Some shaft EAFs also use a charging belt to feed scrap into the shaft furnace. This continuous charging method shortens off-current time and reduces smelting power consumption.
The traditional method for transporting lime from the silo to the EAF station is to use a hopper to transport lumpy lime and other slag-forming materials to the scrap steel yard, where they are then added to a basket filled with scrap steel using an overhead crane.
The new method also allows lime to be transported to the EAF station using a conveyor belt or compressed gas.
Conveyor belt transport can transport lime of varying particle sizes over long distances; however, it carries high initial investment costs, significant lime waste and pollution, and is labor-intensive.
Compressed air delivery, which adds lime powder to the furnace through wall lances, shortens the lime’s movement within the EAF, accelerating lime dissolution and foamy slag formation. However, this requires the use of burners to prevent slag crusting.
The lime powder injection system uses compressed air to transport and add lime powder. The system consists of a silo, injection system, and corresponding piping. Its use reduces lime and dolomite usage per batch by approximately 5%. Adding lime and dolomite via a spray gun allows the operator to automatically control the rate and amount of slag-forming material addition to achieve optimal slag quality.
2. What is the main task of the melting period?
In the electric arc furnace steelmaking process, the melting period, from the moment power is applied until the charge is completely melted, is called the melting period. This period accounts for approximately half of the total smelting time and accounts for approximately two-thirds of total power consumption. The task of this period is to rapidly melt and heat the charge with minimal power consumption, while ensuring the life of the furnace. It also creates a suitable melting slag to stabilize the arc, prevent air absorption, and premature dephosphorization.
Arc starting stage. When the power is turned on and the arc is started, the furnace is filled with charge and the arc is very close to the furnace top. If the input power is too large and the voltage is too high, the furnace top is easily burned. Therefore, the medium voltage and about 2/3 of the rated power of the input transformer are generally selected.
Well penetration stage. In this stage, the arc is completely surrounded by the charge, and the heat is almost completely absorbed by the charge, which will not burn the furnace lining. Therefore, the maximum power is used. Generally, the well penetration time is about 20 minutes, which accounts for about 1/4 of the total melting time.
The electrode rises. After the electrode reaches the bottom of the well, a molten pool forms at the bottom of the furnace. The lime and some elements at the bottom oxidize, forming a layer of slag on the surface of the molten steel. The surrounding charge continues to melt due to radiant heat. The increase in molten steel causes the liquid level to rise, and the electrode gradually rises. This stage still uses maximum power transmission and takes about half of the total melting time.
The final melting stage. After the charge has melted more than three-quarters, the arc is no longer shielded by the charge, and the high-temperature zones under the three electrodes are connected. Prolonged use of high-power electricity at this point can severely damage the furnace cover and walls. The primary task of the melting stage is melting the charge, but creating a good slag is also a crucial operation. To achieve sufficient coverage for the molten steel and stabilize the arc, a slag volume of 1.0% to 1.5% is sufficient. However, to meet dephosphorization requirements, the molten slag must possess a certain level of oxidizability, basicity, and volume.
3. What shape should the molten pool of a modern electric arc furnace adopt?
Traditional electric arc furnace steelmaking uses an electric arc as a heat source. To reduce the high-temperature arc’s erosion of refractory materials, enhance heat conduction within the molten steel, and facilitate the floating of non-metallic inclusions, the melt pool is designed to be shallow and flat, resembling a dish. Modern electric arc furnaces incorporate enhanced oxygen supply and hot-charging technologies, which concentrate the release of a large amount of chemical energy (carbon-oxygen reaction). Continuing to use the traditional electric arc furnace’s melt pool shape not only fails to utilize this concentrated release of chemical energy, but also results in severe splashing, causing heavy slag buildup on the furnace roof and walls, disrupting normal smelting. Furthermore, due to the increased thickness of foamy slag within the furnace, the shallow melt pool shape makes it difficult to control slag release and retention, resulting in low lime utilization and poor dephosphorization effectiveness. By comparing the melt pool shapes of traditional electric arc furnaces (dish-shaped) and those of oxygen-based converters (cup-shaped), which utilize chemical energy for steelmaking, the melt pool shape of modern electric arc furnaces, which utilize a combination of electrical and chemical energy, should be somewhere between traditional electric arc furnaces and oxygen-based converters: a “bowl-shaped” shape.
4. What are the causes of splashing and slag overflow during the melting period?
First, uneven melt pool temperature. This can occur when the melt pool temperature drops to ≤1530°C, causing the carbon-oxygen reaction to cease while power continues. When the temperature rises above 1530°C, the carbon and oxygen react rapidly, producing large amounts of CO gas that overflows, causing splashing and slag overflow.
Second, a surge in metal oxides in the slag can occur. This can occur when improperly added scrap steel forms a bridge, causing the bottom of the accumulated material to empty, leading to a collapse. Carbon and metal oxides react rapidly, producing large amounts of CO gas that overflows, causing splashing and slag overflow.
Third, damp raw materials can cause water to vaporize and overflow at high temperatures, causing splashing and slag overflow. Therefore, it is crucial to use standard raw materials, control the feeding rate (amount) and frequency (time), and promptly address any bridging and accumulation. This can reduce splashing and slag overflow, ensure safety, minimize slag cleaning time, and shorten smelting time.
5.What are the advantages of the wall oxygen supply module technology?
The use of supersonic oxygen jets in electric furnace steelmaking is to promote the carbon-oxygen reaction and utilize its kinetic and chemical energy. This is particularly true when retaining steel and slag or hot charging molten iron. This facilitates earlier oxygen enrichment and enhances bath agitation, thereby shortening smelting time, improving production efficiency, and reducing power consumption.
Conventional electric furnace systems typically utilize three to five injection modules. Each module is equipped with a wall oxygen lance and a carbon injection lance (or secondary combustion burner). The oxygen lance provides fuel fluxing and decarburization, while the carbon lance injects carbon to create foamed slag. With oxygen blowing technology, the oxygen lance jet’s surface area and range are over 30% greater than those of conventional wall fluxing burners (this varies depending on furnace size and installation location). This more effectively accelerates scrap melting and decarburization. The principle is that when the main oxygen jet is directed from the center of the nozzle toward the molten pool, the auxiliary oxygen jet envelops the main oxygen jet, creating a localized “sub-vacuum” state, slowing the decay of the main oxygen jet’s velocity and extending its length.
The concentrated oxygen supply density is approximately 0.8 m³/t·min, and the oxygen pressure in front of the lance must be above 0.8 MPa. Fuel can be either gaseous or liquid, with the amount determined by the charge structure. The carbon powder injection system has an injection capacity of 5 to 30 kg/min, with online adjustment of the injection rate. When the initial carbon content of the molten steel is between 0.3% and 1.2%, the decarburization efficiency of conventional furnace door lances averages 0.04% to 0.05%/min. However, using the concentrated jet, the average decarburization rate can reach over 0.06%/min. Furthermore, the pre-introduction of oxygen significantly widens the decarburization range, making it particularly suitable for applications involving high proportions of molten iron or pig iron, or for smelting low-carbon grades.
6.How to optimize the ingredient structure?
The layout of the scrap steel yard is unreasonable, the site is small, the unloading storage capacity is insufficient, and it is difficult to make a reasonable ratio. In addition to strengthening the management of scrap steel, selection and separation during the pre-processing process, the following issues should be noted: balanced charging, adding larger scrap steel to the middle of the basket to prevent sinking, moderate block size, the widest surface of the solid body shall not exceed 200mm, and the weight of a single piece shall not exceed 500kg, so as not to break the water cooling plate or splash the scrap steel during charging, causing the electric arc furnace to stop working and heat loss; it is forbidden to charge long thin materials and long steel wire ropes to prevent the scrap steel from sticking in the vertical furnace after the two batches of materials are added; for a 100t electric furnace, the amount of pig iron added should not exceed 35t, and 15t should be added to the middle and lower parts of the first and second baskets respectively. The baskets should not be piled together to avoid increasing the melting time; by optimizing the charging quality, the distribution method and the charging process, the possibility of material collapse is reduced, and the negative impact on production is alleviated.
7.What are the power saving measures during the melting period?
Different voltages and currents should be used for each stage of the smelting process, maximizing the transformer’s power supply capacity during the smelting process to maximize arc power.
Melting Period: The melting period accounts for approximately 50%-60% of the total smelting time and 60%-70% of total electricity consumption. Using maximum power can shorten smelting time and conserve energy.
During the initial 5-10 minutes of smelting, medium voltage and high current should be used to prevent the arc from directly radiating onto the furnace roof. After “drilling,” long arc smelting should be performed at the highest voltage and maximum current.
In the later stages of the melting process, after most of the charge has melted, medium voltage and high current should be used to minimize long arc radiation. Oxygen Blowing: Oxygen blowing during scrap melting can increase smelting speed. After the charge reaches a certain temperature, the oxygen blown into the charge reacts with elements such as Si, Mn, C, and P, releasing a large amount of chemical reaction heat, accelerating the melting process. Coal-oxygen burner fluxing.
To eliminate the three cold zones caused by solely melting scrap steel with electrodes and increase smelting speed, a coal-oxygen fluxing process can be implemented in electric arc furnaces. Using a coal-oxygen burner fluxing process can shorten the melting period by approximately 25 minutes and save approximately 75 kW·h/t of electricity. In practice, due to the complex furnace environment, operators must carefully time the ignition and combine it with fluxing to achieve optimal results. The total time savings is approximately 30 minutes. Secondary combustion. During oxygen blowing, carbon in the melt pool is partially oxidized to CO, which has a chemical energy of approximately 3 kW·h/m³ of CO. When oxygen is blown into the melt pool, the CO is oxidized to CO₂, providing a low-cost energy of 5-8 kW·h/m³ of O₂. The carbon loss during oxygen blowing is still estimated at 0.15%.
8.How to supply power when smelting stainless steel?
Because the slag used in stainless steel smelting is extremely viscous, the foaming effect cannot be compared to that achieved during carbon steel smelting. After the scrap is essentially melted, low-voltage, high-current, short-arc operation is employed. However, at the end of the melt, the molten steel temperature is low, resulting in poor foaming, which can reduce active power input.
During production, taking into account the electrical characteristics of the furnace power supply circuit, low-power arc starting and high-power well penetration are employed. Once the molten pool is formed, high current is used for rapid temperature increase.
To optimize the power supply mode, three-phase balance in the furnace is crucial. Increasing active power can effectively shorten furnace smelting time, but energy utilization (thermal efficiency) must be considered when increasing the furnace’s active power. Only when active power and energy utilization are well matched can furnace productivity be truly improved. A reasonable power supply mode must comprehensively consider both electrical and thermal efficiency. Electrical efficiency is related to the transformer itself and the selected operating point. Thermal efficiency, on the other hand, is extremely complex. Energy utilization is closely related to the power supply mode, charge structure, equipment conditions, and foamy slag.
9.How to perform oxygen blowing and fluxing according to different furnace ages?
Oxygen blowing is used to accelerate charge melting and shorten melting time. However, there are two different theories and methods for achieving this accelerated melting. One theory posits that rapidly opening and expanding the three-phase melt well is crucial to quickly create foamy slag, maximize transformer power, and accelerate charge melting. Therefore, oxygen blowing should begin in the center, allowing the three-phase melt well to connect and form a molten pool. Another theory holds that the furnace has high-temperature and low-temperature zones, where charge melts naturally while charge in the low-temperature zone is difficult to melt. Therefore, oxygen blowing should prioritize melting the charge in the low-temperature zone, specifically around the furnace walls. In the early stages of a furnace’s life, the charge load is low, the furnace walls are thick, and the charge is close to the hot spot, resulting in a less pronounced temperature difference and less likely to stick. Therefore, when blowing oxygen to assist melting, the furnace door can be opened directly to clear the charge in the electrode area, allowing the temperature to rise rapidly. Finally, the charge around the edges can be appropriately cleared. In the later stages of a furnace’s life, oxygen can be used to first open the furnace door opening, then blow away the charge at the edge of the furnace wall between No. 3 and No. 1, then blow away the charge from the No. 2 furnace wall to the edge of the taphole, and finally blow away any remaining charge in the center of the furnace. In the later stages of a furnace’s life, it’s important to prevent boiling molten steel from sticking to the charge, which could result in a poor meltability of low-carbon charge. Therefore, the appropriate oxygen blowing method should be used according to the furnace’s age to achieve optimal results.
10. How to improve the life of an electric furnace after hot metal treatment?
Increasing the oxygen lance jet intensity changes the distribution compared to the previous distribution. Increased iron addition and the conversion of the electric furnace increase the Si content in the hot metal. Low initial alkalinity will exacerbate the erosion of magnesia-carbon bricks in the working layer. Therefore, controlling the slag composition is key to improving furnace life. Early slag preparation and increasing the MgO content in the early stages of smelting can prevent MgO from diffusing into the molten pool. Increasing the amount of dolomite, adding an appropriate amount to the initial and final slags, with a total amount exceeding 8 kg/t, or using high-purity magnesium balls (71% MgO content) to partially replace dolomite can increase the MgO content in the slag from 5% to approximately 8%, reducing slag erosion on magnesia-carbon bricks. Producing foamy slag with an FeO content of approximately 25% is crucial. Focused gunning and maintenance are also required on the slag lines on the opposite side of the shaft furnace, below the hot metal nozzle, and below the oxygen lance. Leaks should be promptly addressed to ensure proper operation of the furnace lining under the new process conditions and improve furnace life.
11.What are the classifications and characteristics of modern electric arc furnace oxygen lances?
The increase in the diameter of the electric arc furnace shell has led to more pronounced temperature unevenness within the furnace. Scrap in the center and around the three-phase electrodes melts faster, while scrap in other areas melts more slowly.
Earth arc furnace oxygen technology has evolved from early functions such as fluxing scrap and heating the melt pool to include melt pool stirring, accelerating metallurgical reactions, and secondary combustion.
Earth arc furnace oxygen technology is moving towards multifunctional integration, automated, and efficient control. Electric arc furnace oxygen (oxygen-fuel) lances are categorized by structure as single-hole, double-layer with annular seams, and three-layer. They are also categorized by water cooling type as no water cooling, single-layer water cooling, and double-layer water cooling. They utilize media such as pulverized coal, pulverized carbon, coke dust, graphite powder, methane, acetylene gas, coal gas, natural gas, light diesel oil, and heavy oil, resulting in various configurations, including coal-oxygen lances, oxygen-oil lances, oxygen-natural gas lances, oxygen-coal-gas lances, pure oxygen lances, and composite cluster oxygen lances (including pulverized carbon, oxygen, and natural gas) (see Figure 1). Based on the location of the flame, oxygen lances are categorized as internal combustion and external combustion (also known as internal mixing and external mixing). Based on their installation location, they can be divided into furnace door oxygen lances, furnace wall oxygen lances, EBT oxygen lances, furnace roof oxygen lances, and bottom blowing oxygen lances. Oxygen-oil lances are further categorized as oxygen-light diesel lances and oxygen-heavy oil lances based on the oil quality.
The multi-layer structure of the oxygen lance adds a layer of surrounding gas to the outer core jet. This outer annular flame protects the core jet, maintaining a certain intensity over a longer distance. With rising energy prices, clustered oxygen (fuel) lances are gradually replacing oxygen-oil lances. Because composite clustered oxygen lances can both foam slag and assist in fluxing, they are increasingly used in electric arc furnaces. Coal-oxygen lances were introduced earlier. Compared to layered combustion, pulverized coal combustion offers higher combustion temperatures and greater efficiency.
12.What are the factors that affect electrode consumption?
With the successive implementation of new technologies such as furnace door oxygen lances, oil-oxygen fluxing, EBT cluster oxygen lances, and furnace wall oxygen lances, the oxygen supply within the furnace has increased, creating a stronger oxidizing atmosphere and further increasing electrode consumption. Electrode consumption can be categorized as normal and abnormal. Normal consumption is further divided into tip consumption and side consumption. Abnormal consumption primarily refers to electrode breakage and damage caused by external forces (e.g., failure of threaded connections). Tip consumption is primarily caused by graphite sublimation at high temperatures and melting in slag. Under normal operation, tip consumption can account for up to 50% of total electrode consumption. Electrode oxidation is the primary cause of side consumption, accounting for approximately 40% of total consumption. The oxidation reaction rate is closely related to temperature.
① Within the temperature range of 550-750°C, the oxidation reaction rate is controlled by the electrode itself, with graphite quality and temperature having a stronger influence on electrode consumption than air.
② Above 800°C, air flow rate begins to control the reaction, with air flow rate and pressure having a stronger influence on electrode consumption than temperature and electrode quality. The larger the contact area between the electrode and the air, the greater the intensity of the oxidation reaction and the higher the consumption.
13. What are the reasons for electrode breakage due to internal quality?
According to the internal and external factors affecting the smelting process, electrode breakage can be classified into the following three categories: operational reasons, control reasons, and electrode quality reasons.
(1) Reasons for joint breakage The joint plays a key role in connecting the electrodes during steelmaking. The quality of the joint is directly related to the use of the electrode during electric furnace steelmaking. The connection area formed by the graphite electrode and the joint is a complex area with large electrical, thermal, and mechanical loads, and is also a common fracture area. According to relevant data, in electric furnace steelmaking, more than 80% of electrode use accidents are caused by joint breakage or loosening. As for the joint quality itself, the main reasons for the breakage are as follows: the joint volume density is low, the strength is generally low, and it is easy to cause breakage during use; the resistivity is high, and the temperature of the joint rises rapidly when power is turned on, which will cause the joint to have a large thermal stress at the electrode connection and increase the probability of breakage; the joint’s flexural strength is insufficient; the internal crack joint is mixed into the finished joint, forming a major hidden danger in use; the joint and electrode processing accuracy indicators are not reasonably matched, which is also prone to breakage.
(2) Reasons for electrode breakage Usually, the probability of electrode breakage is low. The main reasons for electrode breakage are as follows: quality defects in the electrode screw hole; insufficient volume density and strength of the electrode; mismatch between the electrode and joint indicators and processing accuracy; deep cracks at the electrode end are caused by the poor thermal shock resistance of the electrode; in addition, electrodes with internal transverse cracks are mixed into the finished product without being detected, which poses a great risk of breakage.
14. What are the causes of electrode breakage?
There are five main types of graphite electrode consumption during the smelting process: electrode tip volatilization, electrode sidewall oxidation, electrode breakage, electrode tip loss, and electrode tip spalling. Among these, electrode breakage, surface spalling, and chipping are collectively referred to as abnormal consumption, to distinguish them from chemical reaction losses, primarily due to oxidation.
Analysis of the cause of the broken electrode (the electrode broke due to contact with scrap steel).
(1) Electrical aspects (unstable electrode movement): The electrical components on the proportional valve amplifier board are short-circuited due to water, the electro-hydraulic servo valve coil and the terminal are loose or desoldered, and the electro-hydraulic servo valve coil oscillation current is not adjusted properly.
(2) Mechanical aspects (electrode lifting system): Improper zero adjustment of the electro-hydraulic servo valve core, internal and external leakage of the hydraulic system, leakage of the electrode lifting cylinder seal, etc.
(3) Other aspects: The charge directly below the electrode is not conductive, resulting in a high probability of the electrode being unable to arc and breaking the electrode. The residual steel on the pole core circle on the furnace cover causes the electrode of this phase to be grounded through the furnace cover and has no voltage, thus breaking the electrode. In addition, improper adjustment of the electrode column guide wheel, loose positioning of the column and the conductive cross arm, and residual steel on the pole core circle that reduces the electrode hole diameter can all cause the electrode to deviate too much due to the poor rigidity of the column and the cross arm when tilting the furnace, thereby breaking the electrode.
15.What are the reasons for electrode breakage due to improper operation?
(1) Collapse of materials and breaking of electrodes
If the scrap steel in the furnace has formed a bridge structure during the smelting process, especially when it is close to being melted, long-term short arc operation will generate a large lateral impact force that causes collapse of materials; an important feature of such accidents is that the torque acting on the electrode clamping point is the largest, and the probability of breaking near the electrode clamping point is the highest. If the electrode end structure is loose or there are dark lines on the hole wall, the joint and the hole are not properly matched, or the linear expansion coefficients of the materials are not matched, the area where the breakage occurs is mostly concentrated in the joint connection area closest to the electrode clamping point. However, when the electrode is loose at a certain point, the probability of breaking at the junction is also high because oxidation accelerates the thinning of the electrode.
(2) Resonance breaks the electrode
Each phase current of the AC arc furnace will generate a magnetic field and is always affected by electromagnetic force. The magnitude of this electromagnetic force is proportional to the electrode current and inversely proportional to the distance between the three-phase electrodes. The attraction or repulsion between the electrodes generally manifests itself as a torque action on the electrode with the electrode holder as the fulcrum. Electromagnetic forces acting on the electrodes during the smelting process inevitably cause them to vibrate. If the mechanical vibration frequency of the electrode column is close to or synchronized with the electromagnetic vibration frequency, the electrode will resonate, which can easily lead to fatigue fracture. Electrode (joint) fractures caused by this type of cause often follow no specific pattern and often occur where the electrode is loose. This type of fracture often occurs at the joint near the middle of the electrode column, primarily due to the relatively large radial temperature difference and high internal stress at this location.
(3) Improper clamping operation leads to electrode breakage.
If the electrode and the clamp cannot be kept perpendicular, an additional horizontal force will be applied to the electrode. If there is foreign matter on the clamping surface, stress concentration is likely to occur at the clamping point. If the electrode is poorly connected, the mechanical strength of the electrode at the connection interface cannot meet the force requirements. These improper operations can easily cause the electrode to break during the smelting process.
16. What are the causes of electrode breakage due to improper control?
① When a non-conductive object is present below the electrode in the furnace, during the electrode’s descent, if the lower end of the electrode contacts this object, the electrode regulator cannot accurately determine that the electrode has reached the correct position. However, the regulator continues to control the electrode to descend, causing it to break due to longitudinal compression against the scrap steel. The fault characteristics are as follows:
a. If the first-phase electrode contacts a non-conductive object, no arc will form, and the electrode will break directly. During this process, the voltage will not decrease.
b. If the second-phase electrode contacts a non-conductive object, the same applies as in a.; no arc will form, and the electrode will break directly. During this process, no current will be detected.
c. If the third-phase electrode contacts a non-conductive object, the first and second-phase electrodes will arc, but the third-phase electrode will not arc and will break directly.
② The electrode regulator cannot detect a drop in voltage to ground for that phase. Under normal circumstances, when a phase electrode automatically descends and its lower end contacts the scrap steel in the furnace, the secondary voltage of that phase should drop to below 20% of the open-circuit voltage or lower. However, if the connection line between the neutral point of the voltage transformer and the furnace bottom shell is disconnected, the electrode regulator cannot detect a drop in voltage to ground for that phase and cannot determine that the electrode has descended to the correct position. This causes the electrode regulator to control the electrode to continue descending, resulting in the electrode being squeezed and broken by the scrap steel. The main characteristic of this fault is that the first electrode always breaks before an arc is formed. After the first phase electrode descends and contacts the scrap steel, the secondary voltage of that phase electrode remains basically unchanged.
③ The electrode regulator cannot detect an arc current. Under normal circumstances, when a phase electrode automatically descends and its lower end contacts the scrap steel in the furnace, the secondary voltage to ground for that phase will immediately drop to below 20% of the open-circuit voltage. The phase electrode will immediately stop descending and wait for the second phase electrode to descend until an arc is formed. If the electrode arc ignites but the electrode regulator cannot detect the arc current, or the arc current is very small, the electrode regulator will control the second phase electrode to continue descending, leading to the second phase electrode breaking due to pressure from the scrap steel. The main characteristic of this fault is that the second or third electrode always breaks after arc ignition.
④ Abnormal condition of the electrode regulator’s hydraulic drive mechanism: During the automatic descent of the electrode, when the lower end of a certain phase electrode contacts the scrap steel in the furnace, that phase electrode should immediately stop descending. If the braking force of the hydraulic drive mechanism decreases or the system’s delay coefficient is set too high, the immediate action of the electrode actuator will be delayed, and the electrode tip may collide significantly with the scrap steel, causing the electrode to break. This type of electrode breakage fault is random. When the electrode hydraulic drive mechanism is abnormal, the main characteristic of this fault is that during the smelting process, the three-phase electrode load current exhibits extreme imbalance, instability, and large fluctuations.
17. How to reduce electrode consumption?
① Structural defects or insufficient strength must be eliminated; otherwise, joint breakage will occur within 1-3 minutes of energizing the first batch of material.
② The machining precision of the electrode end faces is also crucial. Gaps will allow air to pass through, accompanied by localized reddening. After approximately 10 minutes of energizing, the connection will become noticeably red. After 2-3 consecutive heats, the inner thread is prone to oxidation, leading to breakage or detachment.
③ Tolerance fit issues must be constantly monitored. Whether it’s looseness or improper connection, any gap will result in breakage or detachment.
(2) Strict Control of Power Supply System for Energy Consumption Reduction: Strict control of the power supply system is a prerequisite for ensuring normal smelting. Different steel grades and smelting processes have corresponding arc-starting voltages and current settings to prevent excessive current fluctuations and strictly control the peak current operating time. This effectively prevents electrode red-hotness, reactive oxidation consumption, and furnace breakage.
(3) Standardized Use for Energy Consumption Reduction: In principle, large-diameter electrodes must be connected below the furnace. During connection, avoid impacts that could cause thread breakage and ensure the upper electrode remains vertical at all times. Tighten evenly to 8-10mm, then use inertia to lock, and then apply a pre-tightening torque with a long-arm wrench until it cannot be tightened further. Electrodes with seams should not be placed in the furnace. If the phase sequence is correct and the connection is good, it is best not to use fixing pins. In addition, when the flame is high, the uppermost electrode should be connected and heightened in time to prevent damage to the threads; when sliding the electrode, a flexible connection must be used to avoid hard collisions; the holder and the electrode must be kept perpendicular to prevent the electrode from scraping against the small furnace cover; avoid the oxygen lance pointing directly at the lower support electrode inside the furnace, etc.
18. How can we adjust the material composition and change the spraying time to prevent electrode breakage?
Electrode breakage is more frequent with cold furnaces and newly replaced electrodes. Because the electrodes have relatively low strength at room temperature, they are extremely prone to breakage during the arc initiation and well penetration stages. To address this, adjustments can be made to the feedstock of cold furnaces, using only thin, lightweight materials and adding coke to the top of the furnace before arc initiation to facilitate arc ignition. For newly replaced electrodes, the spray system should be turned off and only turned on during the next smelting cycle to prevent water from entering the joints of cold electrodes and affecting their strength. The parameters of the electrode adjustment system have also been adjusted and optimized.
19. What are the operating principles and functions of oxygen blowing oxidation during the melting period?
(1) Direct Oxidation: In the initial stage of oxygen blowing, the temperature of the molten steel is low and the oxygen content is insufficient. At the interface between the gas and the molten steel, the oxidation is mainly direct carbon oxidation.
(2) Indirect Oxidation: After the temperature of the molten steel rises and the oxygen content is sufficient, the oxygen blown into the molten pool first oxidizes the iron in the molten steel. The generated FeO then dissolves in the molten steel and, with the mechanical stirring effect of the oxygen bubbles, rapidly diffuses to the reaction zone, causing carbon oxidation, i.e., indirect carbon oxidation. Both direct and indirect carbon oxidation are exothermic reactions. Therefore, directly blowing oxygen into the molten pool can rapidly increase the temperature of the molten steel, and the change is equal to the sum of the heat released during the oxidation of each element in the molten steel.
The role of oxygen blowing during the melting period is twofold: first, the oxidation of elements such as iron, manganese, silicon, and carbon in the steel releases a large amount of heat, which can directly melt the furnace charge; second, it cuts large pieces of cold material far from the high-temperature zone into smaller pieces, increasing the heating surface area of the furnace charge, thereby accelerating melting. When the furnace charge “bridges”, it is easily handled by using oxygen cutting; thirdly, it adds a mobile heat source inside the furnace, thus compensating to some extent for the uneven heating of the three fixed arc point heat sources.
20.What are the oxygen blowing methods and precautions for oxygen blowing during the melting period of furnace smelting?
The oxygen blowing method used in oxygen blowing for smelting is the door blowing method. The general sequence of oxygen blowing operations at the furnace door is as follows: first, cut the charge on both sides of the furnace door; then, melt and cut the charge on the furnace slope. After the charge is cut, it should be immediately pushed or pulled into the molten pool. Large pieces of refractory charge in the molten pool and ferroalloys deposited at the bottom of the furnace are melted last with oxygen blowing. During oxygen blowing, care should be taken to prevent the oxygen flame from touching the furnace bottom and furnace slope. When cutting the charge on the furnace slope, the oxygen nozzle should be inserted below the contact surface between the solid charge and the molten steel, and at a suitable distance from the charge. Oxygen should not be blown directly onto the charge to avoid damaging the slag line.
Two issues should be considered when using oxygen blowing: the start time of oxygen blowing for molten metal and the oxygen pressure. The reason oxygen blowing aids melting is that the oxidation of metal elements releases a large amount of heat. Therefore, oxygen blowing should only be used when the furnace charge reaches a certain temperature and conditions are met for a vigorous oxidation reaction. This is usually done after a molten pool has formed in the furnace and the charge has turned red-hot (above 900°C). Blowing oxygen too early does not shorten the melting time; on the contrary, it increases oxygen consumption and charge loss. Blowing oxygen too late is also detrimental, as it fails to fully utilize the melting-aiding effect of oxygen and therefore cannot significantly shorten the melting time.
When using oxygen blowing, it is necessary to consider both the melting-aiding effect of oxygen and its full utilization. Based on experience, a suitable melting pressure is generally controlled between 6 and 8 atmospheres. Too high a pressure makes operation difficult to control, reduces oxygen utilization, and may also damage the slag line and furnace slope.
